Interpretation of the postpeak in iron fluorides and oxides

A broad post-edge peak (postpeak) in the core-loss spectrum of a relatively thick TEM specimen is readily accounted for by plural scattering of the transmitted electrons, involving a core-loss and a bulk plasmon. However, we observe a prominent postpeak, about 40 eV above iron L 3 edge, even in very thin films of iron fluoride. The peak gradually disappears as fluorine is removed by electron irradiation, as indicated by decay of the fluorine K-edge and change in white-line ratio, but appears when an iron film is oxidized, therefore it appears to be characteristic of iron in an oxidized state. To study its origin, we performed real-space multiple-scattering (RSMS) calculations of the near-edge fine structure of the Fe L-edge, allowing us to discuss the origin of the postpeak within the framework of electron-scattering theory. In the belief that oxygen and fluorine anions in transition-metal compounds are strong backscatters, we propose that the postpeak is a common feature of transition metals in an oxidized state and can be used as an additional verification of such oxidization in an unknown sample.